Is there a conflict between GPU rendering and the Firefox Browser in 2.8?
I finally worked this out. If the Firefox browser is open on my computer, the GPU render time increases from 6 minutes to 1.5 hours per frame. The solution is to close that browser and then the render times drop again. Don't even have to restart Blender. And once the render is running, I can open up Firefox again although it does seem to add about a minute to the render time.